Am J Sports Med-2011-Coppack-et al

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The American Journal of Sports
http://ajs.sagepub.com/content/39/5/940
The online version of this article can be found at:
DOI: 10.1177/0363546510393269
2011 39: 940 originally published online January 6, 2011Am J Sports Med
Russell J. Coppack, John Etherington and Andrew K. Wills
Trial
The Effects of Exercise for the Prevention of Overuse Anterior Knee Pain: A Randomized Controlled
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The Effects of Exercise for the Prevention
of Overuse Anterior Knee Pain
A Randomized Controlled Trial
Russell J. Coppack,*,z
BSc (Hons), MSc, John Etherington,* MB ChB, MSc, FFSEM (UK), FRCP,
and Andrew K. Wills,y
MSc, PhD
Investigation performed at The Centre for Human Performance, Rehabilitation and Sports
Medicine, Defence Medical Rehabilitation Centre, Headley Court, Surrey, United Kingdom
Background: Anterior knee pain (AKP) is the most common activity-related injury of the knee. The authors investigated the effect
of an exercise intervention on the incidence of AKP in UK army recruits undergoing a 14-week physically arduous training
program.
Hypothesis: Modifying military training to include targeted preventative exercises may reduce the incidence of AKP in a young
recruit population.
Study Design: Randomized controlled trial; Level of evidence, 1.
Methods: A single-blind cluster randomized controlled trial was performed in 39 male and 11 female training groups (median age:
19.7 years; interquartile range, 17-25) undergoing phase 1 of army recruit training. Each group was randomly assigned to either an
intervention (n = 759) or control (n = 743) protocol. The intervention consisted of 4 strengthening and 4 stretching exercises com-
pleted during supervised physical training lessons (7 per week). The control group followed the existing training syllabus warm-up
exercises. The primary outcome was a diagnosis of AKP during the 14-week training program.
Results: Forty-six participants (3.1%; 95% confidence interval [CI], 2.3-4.1) were diagnosed with AKP. There were 36 (4.8%;
95% CI, 3.5-6.7) new cases of AKP in the control group and 10 (1.3%; 0.7-2.4) in the intervention group. There was a 75%
reduction in AKP risk in the intervention group (unadjusted hazard ratio = 0.25; 95% CI, 0.13-0.52; P .001). Three participants
(0.4%) from the intervention group were discharged from the military for medical reasons compared to 25 (3.4%) in the control
group.
Conclusion: A simple set of lower limb stretching and strengthening exercises resulted in a substantial and safe reduction in the
incidence of AKP in a young military population undertaking a physical conditioning program. Such exercises could also be ben-
eficial for preventing this common injury among nonmilitary participants in recreational physical activity.
Keywords: anterior knee pain; injury prevention; exercise; military training
Overuse anterior knee pain (AKP) is a diagnosis that
encompasses several conditions, including patellofemoral
pain syndrome (PFPS). It is typified by pain at the front
of the knee31
and aggravated by activities causing repeti-
tive and high patellofemoral compressive forces such as
kneeling, squatting, and running.9,11,31
Anterior knee
pain is the most common knee disorder and is a major
problem for physically active individuals.36
It accounts
for 25% to 40% of all knee problems presenting to sports
medicine clinics and affects 1 in 4 of active populations.3,15
Military recruits undergoing initial training experience
abrupt increases in physical activity and are thus a high-
risk population for developing AKP.4,17,33
An incidence of
8.7% has been reported in British Army recruits33
and 5%
to 15% in non-UK military recruits.7,21,24
It is the biggest
cause of attrition from recruit training17
and has a debilitat-
ing effect on sufferers, curtailing or ending participation in
physical activity.5
Prevention of such injuries is thus increas-
ingly important in military and civilian populations.26
There is evidence from randomized controlled trials that
a multifaceted prevention program can reduce the incidence
of knee and ankle injuries in specific sports1,13,26
and that
strengthening and stretching exercises can reduce the inci-
dence of lower limb overuse injuries in military recruits.4
Evidence also supports the use of targeted exercise for the
z
Address correspondence to Russell J. Coppack, Centre for Human
Performance, Rehabilitation and Sports Medicine, Defence Medical
Rehabilitation Centre, Headley Court, Epsom, Surrey, KT18 6JW, Eng-
land, UK (e-mail: russ.coppack916@mod.uk).
*Centre for Human Performance, Rehabilitation and Sports Medicine,
Defence Medical Rehabilitation Centre, Headley Court, UK.
y
Medical Research Council Unit for Lifelong Health and Ageing, Uni-
versity College London, London, UK.
The authors declared that they had no conflicts of interest in their
authorship and publication of this contribution.
The American Journal of Sports Medicine, Vol. 39, No. 5
DOI: 10.1177/0363546510393269
Ó 2011 The Author(s)
940
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therapeutic treatment of AKP.8,15,18
The rationale includes
the restoration of patellar alignment and function through
quadriceps strengthening, retraining, and stretching.25,34,35
To our knowledge, there are no randomized controlled trials
examining the effect of these treatment-based protocols spe-
cifically on the prevention of AKP.
We conducted a cluster randomized controlled trial in
a military recruit population undergoing a 14-week struc-
tured and incremental physical conditioning program.
Our aim was to investigate the effect of a targeted exercise
intervention on the incidence of AKP during this program.
MATERIALS AND METHODS
Ethical approval was granted by the Ministry of Defence
(UK) research ethics committee. All participants gave
written informed consent. This study is registered at
http://controlled-trials.com, number ISRCTN61493628.
Study Design and Participants
We used a single-blind cluster randomized controlled
design. All British Army recruits who enlisted at the
Army Training Centre (Pirbright, UK) between July 2006
and February 2007 and passed the entry medical examina-
tion were invited to participate in the study (n = 1502); none
declined. Before enlistment, recruits were divided into
single-sex troops; these were our units of randomization.
Each of the 50 troops (clusters) (average number of recruits
[range]: 41 [22-48]) were randomly assigned to 1 of 2 groups:
an AKP prevention training program (PTP) or a control
program. A simple randomization procedure based on a
computer-generated table of random numbers was used to
allocate the intervention, and an external administrator
provided the group assignment. Physicians diagnosing
AKP cases were blinded. An attempt was made to blind par-
ticipants, but given the physical nature of the intervention,
we refrain from calling this a double-blinded study.
Procedures
All participants were initially screened by a general practi-
tioner and deemed fit to commence training. Baseline
height, weight, and body mass index (BMI) were recorded,
and participants completed a self-administered question-
naire to obtain a history of lower limb injury, exposure to
strength training and running before enlistment, and cur-
rent and past smoking habits. Baseline fitness was also
assessed using a 2.4-km timed running test.
All participants undertook the same 14-week phase 1
basic military training consisting of 3 to 4 hours of formal
daily training activity. Key elements included endurance
marching with backpacks (15%), military field exercises
(24%), running (4%), weapons and foot drill (7%), strength
and conditioning (3%), and theoretical classroom lessons
(23%). Because all recruits followed the same common mil-
itary syllabus with the same daily schedule and environ-
mental conditions, all extrinsic risk factors that may
influence AKP other than the intervention sessions were
the same across the study arms.
The PTP and control sessions were completed during
each physical training lesson of the common military sylla-
bus. All sessions were supervised by qualified army physical
training instructors (PTIs). Participant blinding was
attempted through the application of dummy warm-up exer-
cises for control group participants. Participants in different
groups did not attend concurrent physical training sessions
and were instructed not to reveal information about ses-
sions to the AKP outcome assessor (physiotherapist).
Intervention and Control Program
Fifteen PTIs instructed and supervised the prevention and
control protocols. The instructors were familiarized with
the intervention during a 1-day training workshop. No
training was required for the control program. Written,
standardized program instructions were also provided.
The PTP consisted of 8 exercises (Table 1), delivered in
subsets of 4 during the warm-up and warm-down of each
formal physical training lesson (mean = 7 per week).
Warm-up exercises comprised closed kinetic chain (CKC)
quadriceps and gluteal strengthening exercises. Each exer-
cise was performed in sets of 10 to 14 repetitions. Isometric
hip abduction repetitions were measured in seconds. Partic-
ipants were encouraged to focus on the quality of their
movement, with emphasis on the position of the hip and
knee in relation to the foot. These functional exercises
invoke a strong eccentric contraction of the hip and knee
extensors.5,8,25,35
During the training period, the exercise
load increased progressively as the number of repetitions
increased.
Previous research has shown that poor muscle flexibility
is a predisposing factor for the development of AKP.37
Four
soft tissue stretching exercises were undertaken during the
warm-down of the PTP (Table 1). We used the static stretch-
ing method, 3 repetitions of a 20-second stretch. The quad-
riceps were stretched in the upright standing position. The
stretch was applied by maximal flexing of the knee with the
hip slightly extended. The hamstrings were stretched in
a supine position by lifting the extended leg while control-
ling movement of the pelvis. The gastrocnemius muscle
was stretched in a standing position while leaning against
a wall 0.60 m (2 feet) away and keeping the tested leg fully
extended behind the contralateral side. Iliotibial band
stretches were performed in a supine position by crossing
the flexed knee over the contralateral leg and placing the
foot on the floor. A gentle adduction force was applied to
the flexed knee until the lateral thigh was stretched. The
combined duration of warm-up and warm-down PTP exer-
cise was 15 minutes. For practical reasons, body weight–
resisted exercises were used (see Appendix, available in
the online version of this article at http://ajs.sagepub.com/
supplemental/). Previous research recommends the use of
technically less demanding exercises to enhance compliance
and accommodate the low training experience of recruits.4
The control group undertook existing common military
syllabus warm-up and warm-down exercises, including
Vol. 39, No. 5, 2011 Effects of Exercise for Preventing Overuse Anterior Knee Pain 941

slow running, general upper and lower body stretching,
abdominal curls, and pushup drills. These exercises alone
would be unlikely to cause AKP and were the same dura-
tion as the PTP. The amount of basic training conducted
was not influenced by the introduction of either program.
Although attendance at physical training sessions is com-
pulsory (excluding injury, sickness, or administrative rea-
sons), compliance was assessed using attendance records.
Outcomes
The primary outcome was an incident case of overuse AKP
occurring during the 14-week training period. Anterior
knee pain cases were captured through self-presentation
of pain at the army medical center. Participants were ini-
tially seen by a medical officer. Five experienced physio-
therapists who were blinded to group assignment made
a diagnosis of AKP. Before the study, a consultant
physician (J.E.) delivered a specific training session to the
5 outcome assessors on the diagnosis of anterior knee
pain. The inclusion criteria for an AKP diagnosis were (1)
anterior or retropatellar knee pain arising from at least 2
of the following: prolonged sitting, stair climbing, squatting,
running, kneeling, and hopping/jumping; (2) insidious onset
of symptoms unrelated to a traumatic incident; and (3) pres-
ence of pain on palpation of the patellar facets, on step down
from a 25-cm step, or during a double-legged squat. The
exclusion criteria were signs and symptoms of meniscal or
other intra-articular pathologic conditions; ligament laxity
or tenderness; tenderness over the patellar tendon, iliotibial
band, or pes anserinus tendons; patellar apprehension sign;
Osgood-Schlatter or Sinding-Larsen-Johanssen syndrome;
evidence of a knee joint effusion or hip or lumbar referred
pain; a history of patellar dislocation; or other surgery or
structural damage to the knee. These criteria are sensitive
and specific for diagnosing AKP.8,34
Secondary outcomes were the occupational endpoints of
each participant: successful completion, medical discharge
(MD), discharge as of right (DAOR: a voluntary discharge
at the request of the recruit), unfit for army service
(UFAS: recruits incapable of meeting the training stand-
ards), backsquadding (recruits held back in training), and
other (withdrawal from training for all other reasons).
These were obtained using training group attendance
records and the medical center database. The secondary
outcomes were also used as a marker of adverse events
because we would expect to see rises in backsquadding or
medical discharges if the intervention protocol were caus-
ing other injuries or secondary problems.
Sample Size
The study was designed to detect a 2.5% difference
between groups in the cumulative incidence of AKP. This
was based on reducing the incidence of AKP by 50% in
those cases occurring after day 14 of training (current
knowledge suggests that 10-14 days is the minimum time
required to stimulate a muscular strength and endurance
training adaptation).22
The total estimated sample size,
assuming equal arms, was 2762 participants (type 1 error =
0.05, power = 0.8). Because of military operational commit-
ments, the trial was suspended early, and on termination,
we had recruited 1502 participants. There were no inter-
mediate or data monitoring checks while the trial was
running.
Statistical Analyses
We defined a ‘‘time exposed’’ to training for each individual
and used the hazard ratio as our effect measure. Exposure
time was defined as the length of time an individual spent
in training with his or her original training group free of
AKP. Participants were thus censored at the point they
were removed from training. Reasons for censoring were
a period in rehabilitation, sickness absence, back squad-
ding, an MD from the army, voluntary discharge
(DAOR), or an administrative discharge (UFAS). Partici-
pants who successfully completed training with their
TABLE 1
Anterior Knee Pain Prevention Training Program (PTP)a
Weeks of Program
Exercise 1-3 4-6 7-9 10-12 13-14
Isometric hip abduction against a wall in standing, secb
10 10 15 15 20
Forward lunges—knee over the forward foot, repetitionsb
10 12 12 14 14
Single-legged step downs from a 20-cm step, repetitionsb
10 10 12 12 14
Single-legged squats to 45° of knee flexion with isometric gluteal
muscle contraction, repetitionsb
10 10 12 12 14
Quadriceps (hip and knee) stretches, secc
20 20 20 20 20
Iliotibial band (lateral thigh) stretches, secc
20 20 20 20 20
Hamstring stretches, secc
20 20 20 20 20
Calf (gastrocnemius) stretches, secc
20 20 20 20 20
a
All stretches measured in seconds.
b
Exercises performed during formal physical training session warm-up. All repetitions completed in sets of 3.
c
Stretching exercises performed during formal physical training session warm-down.
942 Coppack et al The American Journal of Sports Medicine

original group were censored at the point of exit (14
weeks). For participants who developed AKP, exposure
was calculated to the point of diagnosis. The Kaplan-Meier
survival function was estimated for the control and inter-
vention arms and plotted and compared using a log rank
test. There was evidence that the baseline hazard of AKP
was not constant when tested formally in a Poisson model,
and thus we used a Cox proportional hazards model to esti-
mate the effect of the intervention. Within-cluster (train-
ing group) correlation was accounted for using robust
standard errors. There was no evidence that the effect of
the intervention differed between sexes (P = .18, Wald
test), so we pooled the sexes for our final models. We pres-
ent an unadjusted and adjusted effect estimate, as the
unadjusted Cox model from a randomized controlled trial
can in certain circumstances produce a biased estimate.
For the adjusted model, we controlled for known predictors
of AKP (smoking, gender, previous injury, previous AKP).
We tested for differences in the secondary outcomes using
a x2
test of general association. Stata v9.2 was used for all
analyses.
RESULTS
Baseline Characteristics
In total, 1502 recruits agreed to participate in the study;
there were no refusals and no exclusions or dropouts before
consent. There were 759 participants (556 [73.3%] men;
203 [26.7%] women) in the intervention group and 743
(536 [72.1%] men; 207 [27.9%] women) in the control arm
(Figure 1). The groups were well balanced at baseline in
terms of height, weight and BMI, social and training his-
tory, and previous injuries (Table 2). The total exposure
time was 2034 recruit training months in the intervention
group and 1824 recruit training months in the control
group. The mean individual compliance rate for the 2 pro-
grams was 91%, and there was no evidence of a difference
between groups (P . .05). There were no reports of pain
from the intervention exercises.
Primary Outcome
Of 1502 participants, 46 (3.1%; 95% confidence interval
[CI], 2.3-4.1) were diagnosed with AKP. The median time
of diagnosis of AKP was 4.3 weeks (interquartile range
[IQR], 3.1-8). Pooling the 2 groups, the incidence of AKP
was 0.012 (95% CI, 0.009-0.016) per recruit-months of
training. In men, the incidence was 0.012 per recruit-
month (95% CI, 0.009-0.017), and in women, the incidence
was 0.011 (95% CI, 0.006-0.020). There was no evidence of
a sex difference in the incidence of AKP (P . .05).
There were 36 new cases of AKP in the control group
(cumulative incidence: 4.8%; 95% CI, 3.5-6.7) and 10 in
the intervention group (cumulative incidence: 1.3%; 95%
CI, 0.7-2.4). The incidence in the control group was 0.020
per recruit-month (95% CI, 0.014-0.027) versus 0.005
(95% CI, 0.002-0.009) in the intervention group. Figure 2
shows the cumulative probability of developing AKP over
the course of training in each arm of the study. There
was very strong evidence for a difference in this survival
function between groups (P .01, log rank test).
There was no evidence of a gender interaction in the
effect of the intervention (P = .18 from the proportional
hazard model), so we report the pooled analysis. There
was very strong evidence of a protective effect of the
intervention. The hazard (risk) of AKP over training
was reduced by 75% (95% CI, 52-87) in the intervention
group compared to the control group, and this is an esti-
mate of the preventable fraction in the control group
(Table 3). The absolute risk difference between the con-
trol and intervention groups was 20.015 per recruit-
month (95% CI, 20.022 to 20.008). The unadjusted and
adjusted hazard ratios for the effect of the intervention
were very similar (Table 3). After adjustment for cluster-
ing, standard errors were also very similar, suggesting
that the observations within each troop were largely
independent.
Secondary Outcomes
There was strong evidence for a difference in the secondary
outcomes between groups (P .001). In the control group,
25 (3.4%) were medically discharged compared to 3 (0.4%)
in the intervention group. Six (0.8%) in the intervention
group were discharged on the grounds of ‘‘unfit for army
service’’ compared to 23 (3.1%) in the control group. The
majority of participants in both groups successfully com-
pleted training, although a greater proportion from the
intervention group had a successful outcome (79% vs
68%), and there was no evidence to suggest a difference
in voluntary discharge (DAOR) rate between groups (P .
.05) (Table 4).
Of the 10 AKP cases in the intervention group, 9 (90.0%)
successfully completed training, and 1 (10.0%) was
UFAS—none were medically discharged. In the control
group, 10 (27.8%) participants with AKP were medically
discharged, 6 (16.7%) were backsquadded, 2 (5.6%) were
UFAS, 2 (5.6%) were voluntarily discharged (DAOR), and
16 (44.4%) successfully completed training.
DISCUSSION
Principal Findings
Our study shows that a simple set of targeted exercises
was effective in producing a large reduction in the risk of
AKP in a young military population undergoing an inten-
sive 14-week physical conditioning and training program.
We also showed a significant and important improvement
in occupational outcomes such as a reduction in discharges
from the service in the intervention group. There were no
adverse effects among individuals undergoing the inter-
vention protocol. As far as we are aware, this is the first
randomized controlled trial demonstrating the use of a pre-
vention protocol for overuse anterior knee pain.

Strengths and Limitations
This study has several strengths. With the exception of the
control/intervention sessions, the daily exercise load and
exposure to other extrinsic risk factors for AKP of each
individual were similar and monitored and supervised.
Because of the military training setting, no individuals were
lost to follow-up. The intervention was specifically targeted
at anterior knee pain, and the sessions were supervised to
ensure correct execution and compliance. There was no
TABLE 2
Baseline Descriptive Characteristics of Intervention and Control Group Participants by
Gender (n = 1502): Pretrial Distribution of Knee, Ankle, Acute, and Overuse Injuries
Demographics and Physical Characteristics
Intervention Group (n = 759) Control Group (n = 743)
Variable Men (n = 556) Women (n = 203) Men (n = 536) Women (n = 207)
Age, y, mean 6 SD 19.4 6 2.1 20.2 6 2.9 19.5 6 2.4 19.8 6 2.8
Height, m, mean 6 SD 1.76 6 0.06 1.64 6 0.06 1.76 6 0.06 1.65 6 0.06
Weight, kg, mean 6 SD 70.7 6 9.6 61.1 6 7.7 69.9 6 9.4 60.9 6 7.8
Body mass index, kg/m2
, mean 6 SD 22.7 6 2.6 22.6 6 2.3 22.6 6 2.5 22.5 6 2.2
Baseline fitness, mean 6 SDa
637.4 6 64.4 754.7 6 64.2 634.2 6 61.1 758.9 6 58.5
Smoking habits, No. (%)b
Nonsmokers 282 (50.7) 129 (63.5) 297 (55.4) 121 (58.5)
Current smokers 274 (49.3) 74 (36.4) 239 (44.6) 86 (41.5)
1-7 73 (13.1) 22 (10.8) 66 (12.3) 26 (12.5)
8-14 121 (21.8) 40 (19.7) 107 (19.9) 44 (21.3)
.15 80 (14.4) 12 (5.9) 66 (12.3) 16 (7.7)
Running, No. (%)c
None 52 (9.4)g
8 (3.9) 51 (9.5) 7 (3.9)
1-5 259 (46.7) 102 (50.2) 236 (44.0) 103 (49.8)
6-10 157 (28.3) 68 (33.5) 177 (33.0) 70 (33.9)
11-15 43 (7.7) 14 (6.9) 38 (7.1) 17 (8.2)
.16 44 (7.9) 11 (5.4) 34 (6.3) 10 (4.8)
Strength training, No. (%)d
No 119 (21.4)g
33 (16.7) 128 (23.9) 39 (18.9)g
1 56 (10.1) 20 (9.8) 42 (7.8) 18 (8.7)
2-3 260 (46.9) 109 (53.7) 255 (47.6) 102 (49.5)
.4 120 (21.2) 41 (20.2) 111 (20.7) 47 (22.8)
Injury Type Intervention Group (Pooled Sex) Control Group (Pooled Sex)
Body part, No. (%)
Knee 78 (10.3) 74 (9.9)
Ankle 108 (14.3) 68 (9.2)
Acute injuries, No. (%)e
Knee ligament injury 10 (1.3) 14 (1.8)
Ankle ligament injury 107 (14.1) 68 (9.2)
Bone fractures 14 (1.2) 4 (0.5)
Meniscus injury 13 (1.7) 7 (1.0)
Other 29 (3.9) 28 (3.8)
Overuse injuries, No. (%)f
Anterior knee pain 14 (1.8) 15 (2.0)
Patellar tendinitis 1 (0.1) 0 (0.0)
Osgood-Schlatter syndrome 12 (1.6) 19 (2.6)
Previous surgery, No. (%)
Knee 9 (1.2) 5 (0.7)
Ankle 14 (1.8) 6 (0.8)
a
Baseline fitness is 2.4 km measured in seconds.
b
Smoking habits is number of cigarettes per day (%).
c
Running is number of miles per week (%).
d
Strength training is number of sessions per week (%).
e
Values are pooled (by gender) numbers (%) of participants. Acute injury defined as occurring with a sudden onset associated with a known
trauma.
f
Overuse injury defined as occurring with a gradual onset without any known trauma.
g
Data missing for one participant.

suggestion that the groups differed at baseline in terms of
risk factors for AKP or factors that may be related to occupa-
tion outcomes (eg, BMI, fitness, smoking, previous injuries).
The method of case capture was first hand rather than rely-
ing on retrospective information such as medical notes.
In our study, the diagnosis of AKP was generic, with no
identification of specific causes or analysis of predisposing
factors for AKP. Consequently, any views on the effective-
ness of our program in combating certain predisposing fac-
tors or preventing specific diagnoses responsible for AKP
would be speculative. Unfortunately, because of military
operational commitments, data collection was stopped early,
and the estimate of the effect lacks precision. However, the
study was still adequately powered because the effect we
saw was larger than hypothesized. It is possible that there
was some censorship bias because of more participants
from the control group being medically discharged; if indi-
viduals who were medically discharged were also more likely
to develop AKP, then the effect would have been underesti-
mated. Blinding of outcome assessors was not completely
successful. In 33% (6 in the intervention group and 9 in
the control group) of all AKP cases, the trained observer
responded that he or she was aware of participant allocation.
In procedural intervention trials this can account for an
overestimation of effect by up to 17%.30
Last, there was no
follow-up beyond 14 weeks, and so no conclusions can be
drawn about the long-term benefits of the intervention.
Strengths and Weaknesses in Relation to Other
Studies and Comparison of Findings
The intervention program comprised well-established
exercises used in the management of AKP.5,8
This regimen
Allocated to PTP intervention group.
21 troops (n = 759)
Received allocated intervention (n = 759)
Did not receive allocated intervention
(n = 0)
Allocated to CP control group.
23 troops (n = 743)
Received allocated intervention (n = 743)
Did not receive allocated intervention
(n = 0)
Analyzed (n = 759 )
Lost to follow-up (n = 0 ) Lost to follow-up (n = 0 )
Analyzed (n = 743 )
Assessed for eligibility (n = 1502)
Randomized (44 troops; 1502 participants)
Figure 1. Trial profile: flow of troops and participants through the study. CP, control program; PTP, prevention training program.
Control (CP) group
Intervention (PTP) group
0
1
2
3
4
5
6
7
8
0 2 4 6 8 10 12 14
Log rank test: χ2
= 17·45 , p<0·001
Cumulative%withAKP
Training time (weeks)
Figure 2. Failure plot for anterior knee pain (AKP) in the control
and intervention groups over the period of training. The top line
is the cumulative % with AKP in the control group, and the bot-
tom line is the cumulative % with AKP in the intervention group.
TABLE 3
Effect of Intervention (Hazard Ratio)a
Hazard Ratio 95% Confidence Interval P Value
Unadjusted 0.25 0.13-0.48 .001
Adjustedb
0.25 0.13-0.49 .001
a
Estimates are from a Cox proportional hazards regression
model with robust standard errors to account for the cluster
design.
b
Adjusted for gender, age, previous ankle injury, previous epi-
sode of anterior knee pain, physical fitness, smoking (cigarettes
per day), body mass index, and height.

has produced beneficial effects in randomized controlled
trials.8,9,18
In contrast to many previous injury prevention
studies, which include several lower limb injuries,4,10,13
we
chose to focus on the prevention of AKP alone.
Several other exercise intervention studies have demon-
strated significant reductions in injury rates in their
trained groups. In these studies, the investigators exam-
ined the effect of balance training,13
multifaceted struc-
tured warm-ups,26
or plyometric jump training.19
The
hazard ratio of injury found in our study (0.25; 95% CI,
0.12-0.51) is of a similar magnitude compared with another
randomized controlled trial examining preventative warm-
up exercises in adolescents (relative risk, 0.35; 95% CI,
0.19-0.63).26
A systematic review of multi-intervention
training programs incorporating strength and balance
exercises also reported injury risk reductions greater
than 50% in 5 of 6 of the studies reviewed.1
In contrast to our findings, 2 recent randomized con-
trolled trials found no preventative effect of an AKP inter-
vention in military recruits4
and elite volleyball players.10
In both studies, compliance rates were poor (75%), and
exercises were performed incorrectly10
or without proper
supervision.4
Furthermore, both studies employed a broad
approach that included other knee or lower limb injuries.
Another randomized trial reporting low compliance rates
in male soccer players found no preventative effects of
exercise.14
In our study, participants performed all exer-
cise sessions under qualified supervision. This ensured
consistency in the quality of exercise technique and high
rates of compliance (90%). On the basis of these findings
and previous studies,13,26
we suggest that supervision is
a potentially key factor in AKP prevention protocols.
Another difference between our study and the only
other randomized AKP prevention trial with military
recruits4
is the type and dosage of exercise. Brushøj
et al4
employed 3 sets of 5 exercises, 3 times per week (total
weekly duration = 45 minutes), for 12 weeks. We employed
3 sets of 8 exercises, 7 times per week (total weekly dura-
tion = 105 minutes), for 14 weeks. A minimum amount of
exercise must be performed before an effect may be
expected.14,22
Depending on the goal of training, the exer-
cise dosage in both studies conforms with published recom-
mendations.22
However, this number is arbitrary as there
is no evidence on the dose-effect relationship for any exer-
cise program to prevent injury.14
Interpretation, Explanation, and Implication
Training programs that address intrinsic risk factors are
a well-established method of reducing AKP inci-
dence.10,32,37
The quadriceps play a key role in the causes
of AKP.5,9
Several studies indicate that AKP patients dem-
onstrate an imbalance in the neuromotor control between
the vastus medialis oblique (VMO) and vastus lateralis
(VL).34,36
Reduced motor activity of the VMO relative to
the VL could cause lateral patellar tracking and subluxa-
tion.5,8,37
The prevention program that we tested incorpo-
rated CKC functional strengthening exercises. These
exercises have been shown to increase VMO and VL activa-
tion patterns6
and improve patellofemoral joint alignment
in healthy adults.20
Previous studies using VMO CKC exercises in AKP
patients have reported positive effects.8,12,25
In our study,
a high percentage of AKP cases was sustained in the first
5 weeks of training (Figure 2). Previous research in UK
military recruits showed a similar temporal pattern.33
Sur-
face electromyographic (EMG) techniques reveal that
strength gains in the early phase of training are associated
with an increase in the amplitude of EMG activity.16,25
Eccentric contractions appear to involve a different EMG
activation scheme to isometric and concentric contrac-
tions,16
and one older study showed relief of AKP after
only 2 to 4 weeks of eccentric training.2
Because our intervention group focused on quadriceps,
pelvis, and hip muscle recruitment, we would expect better
hip and knee motor control during weightbearing activi-
ties.25
It is plausible that improved motor control motion
had an important role in the prophylactic effect seen in
the intervention group. Specifically, movement patterns
were executed more efficiently, thereby reducing the stress
on the patellofemoral joint. The emphasis on controlled
eccentric exercise may have increased the muscular capac-
ity to absorb high patellofemoral forces during the early
stages of training. However, this view is speculative. No
effects of this program have been reported in injury pre-
vention studies, and a recent prospective randomized trial
showed no alteration in reflex response times of VMO and
VL in AKP sufferers using CKC exercises.34
Thus, strong
evidence in support of a neuromuscular adaptation in our
program is lacking, and further research investigating
the potential protective effects of exercise on disturbed
neuromuscular balance is needed.
Stretching of soft tissue structures has been shown to
reduce pain and increase flexibility in AKP patients.5,8,12
This would imply that a stretching program may be bene-
ficial in the prevention of AKP. However, scientific evi-
dence supporting the protective effects of stretching is
lacking. Two randomized studies using military recruits
failed to show any positive effect of stretching on
TABLE 4
Training (Occupational) Outcome by Groupa
Group Code
Training Outcome
Control,
No. (%)
Intervention,
No. (%)
Total,
No. (%)
DAOR 71 (9.6) 76 (10.0) 147 (9.8)
MD 25 (3.4) 3 (0.4) 28 (1.9)
UFAS 23 (3.1) 6 (0.8) 29 (1.9)
Successful completion
of training
504 (67.8) 605 (79.7) 1109 (73.8)
Otherb
120 (16.2) 69 (9.1) 189 (12.6)
Total 743 (100) 759 (100) 1502 (100)
a
Abbreviations: DAOR, discharge as of right; MD, medical dis-
charge; UFAS, unfit for army service. Pearson x2
(4) = 50.2168,
Pr = 0.001.
b
Backsquadded, rehabilitation, sickness.

individual injury risk,27,28
and it is not possible to deter-
mine which elements of our intervention program may be
effective in preventing AKP. Although systematic reviews
report mixed evidence that stretching exercises for injury
prevention are effective,1
the findings of this trial suggest
that abandoning stretching interventions for AKP preven-
tion would be premature.
Our results showed that a reduction in AKP was
reflected in significantly lower MD rates in the interven-
tion group (Table 4). Beneficial results were also observed
for training days lost and completion of training. A previ-
ous study with UK military recruits showed that AKP
cases accounted for 25% of all medical discharges and
took longer to train.33
One non-UK study found that
a 46.6% reduction in all injuries resulted in a 40.0% reduc-
tion in medical discharge rates in male recruits who fol-
lowed a modified exercise program.29
A further study
showed a 14.2% reduction in attrition in recruits undergo-
ing a prevention conditioning protocol.23
These studies
were nonrandomized and suffer from design limitations.
However, combined with our data, they do demonstrate
that reduced injury incidence can significantly improve
occupation outcomes.
Generalizability of Results
Although the age group (17-30) in our study is representa-
tive of the typical case of AKP in the general population,
we do not know if our results can be generalized to other
age groups or to groups with different activity levels. Con-
sequently, our findings are specific to a young military pop-
ulation and cannot be considered a panacea for all AKP
sufferers. If the goal of training is to develop eccentric
motor control patterns that are resistant to injury, it may
be beneficial to introduce our program with younger ado-
lescents who have not yet established their motor pat-
terns.26
Introduction of preventative exercise programs
may also lead to fewer injuries and training hours lost in
athletic groups and lower medical costs among the general
population. However, achieving high compliance rates
among civilian populations who are not subject to manda-
tory training programs could be a problem. Further
research is needed among populations other than military
recruits focusing on specific types of injuries.1
CONCLUSION
A targeted exercise protocol significantly reduced the inci-
dence of AKP and improved secondary occupation out-
comes in military recruits undergoing a 14-week training
program. We recommend that preventative training
should be introduced as a core component of phase 1
army training. It is possible that close supervision of cor-
rect exercise technique by professionally qualified staff
may be a crucial factor in AKP prevention. Future research
should focus on the long-term benefits of preventative
exercise programs and the relative effectiveness of this reg-
imen among nonmilitary and recreational populations.
ACKNOWLEDGMENT
We acknowledge and offer grateful thanks to the medical
center, physiotherapy department, and physical training
wing staff at the ATC Pirbright. Particular thanks are
extended to the sword company clerk, Mrs Lorraine Con-
nelly, for her invaluable help and cooperation during
data collection. We also thank Gethin Owen and Richard
Horsley (DMRC), as well as James Bilzon and Rachel Izard
(ARTD Occ Med), for their support in instigating this
study. We also thank Luisa Zuccolo (University of Bristol)
for her useful comments in reviewing this manuscript.
This work was sponsored by the Army Recruitment and
Training Division (ARTD), UK.
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